One type of thermal printer employs a dye-donor element placed over a dye-receiver element. The two elements together are moved past a print head having a plurality of very small heat "sources". When a particular heating source is energized, thermal energy from it causes a small dot or pixel of dye to transfer from the dye donor element onto the receiver element. The density of each dye pixel is a function of the amount of energy delivered from the respective heating source of the print head to the dye donor element. The individual pixels are printed in accordance with image data; all of the dye pixels thus formed together define the image printed on the receiver element.
Because light from a laser can be focused to an ultra-fine, intense spot of heat energy and can be modulated at very high speed, lasers (such as small, relatively inexpensive diode lasers), are now the preferred heating sources for printing the dye pixels in the more advanced thermal printers. But in the case where pixels are printed at very fine pitch on very closely spaced lines (e.g., 1800 lines per inch and 1800 pixels per inch), it becomes impracticable to provide an individual laser for each line across the width of a page being printed. For example, a 10 inch wide page would require 18,000 lasers and respective drive circuits. On the other hand, using only one laser and scanning in sequence the lines across a page to print an image is a very much slower operation than when multiple lasers are used.
In U.S. patent application Ser. No. 451,655 filed Dec. 18, 1989, now U.S. Pat. No. 5,164,742, entitled "Thermal Printer", and assigned to an assignee in common with the present patent application, there is disclosed a thermal printer employing a plurality of lasers for printing a like plurality of lines of print pixels at the same time. This thermal printer produces full color pictures printed by thermal dye transfer in accordance with electronic image data corresponding to the pixels of a master image. The pictures so produced have ultra-fine detail and faithful color rendition which rival, and in some instances exceed in visual quality, large photographic prints made by state-of-the-art photography. This thermal printer is able to produce either continuous-tone or half-tone prints. In the continuous tone mode, the ultra-fine printed pixels of colored dye have densities which vary over a continuous tone scale in accordance with the image data. On the other hand in the half-tone mode, the ultra-fine print pixels which define the picture are formed by more or fewer micro-pixels of dye such that the pixels printed closely together appear to the eye as having greater or lesser density and thus simulate a continuous tone scale. Half-tone, offset printing is widely used for example, in printing and publishing. It is common practice in this and related industries first to obtain and visually inspect "proof" prints prior to production so that any visual blemishes, artifacts of the half-tone process, or other undesirable qualities in the "printed" pictures, (which would otherwise occur in production) can be corrected before production begins. In the past, the obtaining of these "proof" prints has involved considerable time delay and significant extra expense. This thermal printer, by virtue of its unique design and mode of operation is able to produce quickly (within minutes) an authentic half-tone printed image which (for all intents and purposes) is visually indistinguishable from the highest quality color image made by offset printing. And by comparison, the initial setup costs and processing times for the printing plates required in high quality offset printing are man times (e.g., hundreds) the costs and times required by this thermal printer to produce "proof" prints of equal quality. This not only simplifies the publishing operation prior to production, but helps a publisher improve the visual quality of the end product (e.g., an illustrated magazine).
The human eye is extremely sensitive to differences in tone scale, to apparent graininess, to color balance and registration, and to various other incidental defects (termed printing artifacts) in a picture which may occur as a result of the process by which the picture is reproduced. Thus it is highly desirable for a thermal printer such as described above, when used in critical applications, to be as free as possible from such printing artifacts.
The thermal printer described in the above-mentioned U.S. Patent Application has a rotating drum on which can be mounted a print receiving element with a dye donor element held closely on top of it. The two elements are in the form of thin flexible rectangular sheets of material mounted around the circumference of the drum. As the drum rotates, a thermal print head, with individual channels of laser light beams in closely spaced, ultra-fine light spots focused on the dye element, is moved in a lateral direction parallel to the axis of the drum. With each rotation of the drum, multiple lines (termed a "swath") of micro-pixels are printed on the receiving element in accordance with image data applied to the electronic driving circuits of the respective laser channels. There are as many image lines in a swath as there are laser channels (for example, 18 lines with a lateral spacing of 1800 lines per inch) and there are as many swaths as required to print an image of a given page width. It has been found that even minute differences in the drive power applied to the individual lasers can cause objectionable visual differences in the densities of the micro-pixels printed by the individual laser channels where supposedly equal densities are called for by the image data. Power differences as seemingly unimportant as a small fraction of one percent of a desired laser drive power level are visually noticeable as density variation of the printed pixels. It is therefore highly desirable to be able quickly and easily to make a visual check to show that all of the laser channels are printing uniformly and with proper timing.
The multiple swaths printed by the thermal printer disclosed in the above-identified U.S. Patent Application must be precisely registered side-by-side relative to each other and the individual print lines of a swath must be precisely aligned in time relation relative to each other. Even very slight (e.g., a few microns) mis-registration or mis-alignment causes degradation in the visual quality of a printed image. The print head of this thermal printer is provided with micrometer adjustment of the angular position of the print head and hence of the pitch of the image lines printed within a swath (see FIGS. 2 and 3 of the above-identified U.S. Patent Application). Thus it is possible to mechanically adjust the head to obtain accurate print registration from swath to swath. However, because the adjustments have to be precisely made, it is advantageous to be able to print a visual test image which immediately verifies proper swath registration and alignment of the printed lines.
The thermal printer described above has associated with it various electronic circuits. These circuits include a data interface module (DIM) and a plurality of drive circuits, supplied with line data by the DIM, for driving a like plurality of laser channels of the print head. The DIM receives image data from a raster image processor (RIP) which converts continuous-tone image data at high speed from a computerized editing and proofing station (CEPS) and converts the data on-the-fly to half-tone bit image data, which it supplies to the DIM. Due to the large size of the data files (e.g., 200 megabytes) and the high speed at which the RIP supplies data (e.g., 10 megabits/sec.) to the DIM, software management of the RIP to generate suitable image data for testing and adjusting the printer is cumbersome and expensive. The present invention provides a simple and highly effective solution to this problem of testing and adjusting, cost, etc.